1. Field of the Invention
The present invention relates to the field of x-ray imaging. More particularly, embodiments of the invention relate to methods, systems, and apparatus for imaging, which can be used in a wide range of applications, including medical imaging, security screening, and industrial non-destructive testing to name a few.
2. Description of the Related Art
Since its invention in 1973, X-ray computed tomography (CT) has revolutionized medical imaging and become a cornerstone of modern radiology. Improving resolution and reducing dose are two critical factors in biomedical applications and remain the focuses of CT research. With the emergence of multislice spiral CT in 1998, cone-beam scanning is recognized as a major mode for medical CT and widely used in numerous diagnostic and therapeutic procedures. Moreover, the rapid development of small animal models, especially those with genetically engineered mice, has generated the need for preclinical imaging, reaching image resolution in the micron range. These scanners, while producing high spatial resolution images, do not allow high contrast and low dose imaging in either patients or animal models. For example, many normal and diseased tissues such as cancers display poor image contrast in current X-ray images as they have very similar attenuation characteristics.
X-ray mammography is currently the most prevalent imaging modality for screening and diagnosis of breast cancers. The use of mammography results in a 25%-30% decreased mortality rate in screened women, however, a multi-institutional trial funded by the American College of Radiology Imaging Network (ACRIN) suggested that approximately 30% of cancers were not detected by screening mammography, and 70%-90% of biopsies performed based on suspicious mammograms were negative. Some false negative and false positive diagnoses often led to missed cancers and inappropriate biopsies.
Conventional medical x-ray imaging, such as mammography, relies on the attenuation contrast mechanism. Biological soft tissues encountered in clinical and pre-clinical imaging (such as breast tissue, gray-white brain matter, liver, mouse tissues, etc.), however, consist mainly of light elements. As a result, the elemental composition is nearly uniform without much density variation. Because of the insufficient contrast between the healthy and malignant tissues, some early-stage tumors cannot be identified using attenuation contrast imaging. In some cases, the x-ray attenuation contrast is relatively poor and cannot offer satisfactory sensitivity and specificity, a key factor limiting the success rate in diagnosing diseased tissue.
Specifically for diagnosing breast cancer, although X-ray CT of the breast can potentially improve diagnostic accuracy over mammography, the state-of-the-art breast CT scanner is still based on the attenuation mechanism. As a result, the use of breast CT requires an intravenous contrast medium and a high radiation dose, since elemental composition is almost uniform with little density variation in breast tissues. Still, it is rather difficult for breast CT to discern early-stage breast cancers.
Absorption and scattering are two largely independent properties of an object. Generally speaking, each is an important factor in characterizing an object optically. The literature already reported that scattering coefficients contain important physiological and pathological information for cancer screening and other purposes. The x-ray scattering in the biological tissue provides an effective contrast mechanism for x-ray imaging that may well outperform or effectively complement attenuation-based imaging. Scattering-based imaging can improve or enable diagnosis for early-stage cancer, and has widely applications to soft tissue imagings. By reconstructing both absorption and scattering properties, x-ray CT can be elevated to the next level with major healthcare benefits.
Driven by major practical needs for better X-ray imaging, exploration into contrast mechanisms other than attenuation has been active for decades, especially in terms of small angle scattering (essentially, Rayleigh scattering) and refraction of X-rays, which are also known as dark-field and phase-contrast imaging, respectively.
Up to now, X-ray Rayleigh scattering-based imaging has been limited to in vitro studies, incapable of volumetric cone-beam scanning, lack of rigorous reconstruction theory, and made little progress into clinical practice.
Since 2006, grating-based X-ray dark-field and phase-contrast tomography is being developed using a hospital-grade X-ray tube, instead of a synchrotron facility or microfocus tube. More specifically, for example, Pfeiffer and coworkers proposed a grating interferometer technique to produce dark-field images using a hospital-grade x-ray tube. F. Pfeiffer, M. Bech, O. Bunk, P. Kraft, E. F. Eikenberry, Ch. Brönnimann, C. Grünzweig, and C. David, “Hard-X-ray dark-field imaging using a grating interferometer,” Nature Materials 7, 134-137 (2008). This technology utilizes the optical interference principles to yield high quality dark-field images. The boundaries and interfaces in the biological tissues produce strong signals in dark-field images, indicating detailed structural contours. Moreover, dark-field images have greater signal-to-noise ratios in soft tissues than bright-field counterparts acquired with the same incident X-ray dose. However, the major problems with this grating-based approach are small sample size, long imaging time, and high fabrication cost.
Such an imaging modality may greatly enhance sensitivity and specificity for soft tissue imaging, revealing subtle structural variation of tissues. However, the data acquisition procedure is quite time-consuming. The gratings with large sizes and high slit aspects are difficult to fabricate and model, especially since the analyzer absorption grating consists of Au pillars encased in epoxy and bounded using a frame.
In 2004, Harding proposed an x-ray coherent scattering imaging method. It uses an x-ray fan-beam to illuminate an object slice for acquisition of coherent scattering data with multiple detector rows. G. Harding, “X-ray scatter tomography for explosives detection,” Radiation Physics and Chemistry 71, 869-881 (2004). The central detector row of this technology receives the transmitted radiation while the out-of-center rows record only scattered radiation. The technique is able to perform a rapid scan of the object and provides a significant increment in image contrast for quantitative analyses. However, scattering cross-talks cannot be avoided in this imaging modality and would significantly degrade image quality. Efficient and high-quality acquisition of x-ray small-angle scattering signals is still a challenge.
Further, for example, to perform tomographic imaging from x-ray small-angle scattering signals, Strobl et al. proposed a method to simulate the broadening of the angular distribution of small angle scattering for dark field tomographic imaging. This broadening is related to both microscopic structure and multiple scattering along the path length through a matter. Strobl, M., W. Treimer, and A. Hilger, “Small angle scattering signals for (neutron) computerized tomography,” Applied Physics Letters, 85, 488-490 (2004); and M. Strobl, C. Grünzweig, A. Hilger, I. Manke, N. Kardjilov, C. David, and F. Pfeiffer “Neutron dark-field tomography,” Physical Review Letters 101, 123902 (2008).
Harding and coworkers approximate the small-angle scattering propagation as a linear model and directly used the filtered backprojection algorithm to reconstruction scattering contrast images from dark field data. See, G. Harding (2004). However, the propagation of x-ray photons through matter is a complex process, which experiences both absorption and scattering simultaneously. A photon propagation model describes photon interaction with matter, and is essential for tomographic imaging.
Thus what is needed is an imaging modality having sufficient sensitivity and specificity to provide high contrast, high quality x-ray based images. Provided by embodiments of the invention are several novel approaches and associated systems for x-ray small-angle scattering based imaging to produce high-contrast images.